Mobile communication devices such as cellular phones commonly employ heterojunction bipolar transistors (HBTs) in order to amplify the power of radio frequency (RF) signals transmitted to base stations. For example, Japanese Unexamined Patent Application Publication No. 2002-76015 discloses an HBT in which a collector layer, a base layer, and an emitter layer are laminated in this order, and the emitter layer has a doping concentration that is uniform throughout the emitter layer.